EP2685177B1

EP2685177B1 - Heat pump-type water heater

Info

Publication number: EP2685177B1
Application number: EP12755362.6A
Authority: EP
Inventors: Hideo Chikami; Yuuichi Kita; Hidehiko Kataoka
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 2011-03-10
Filing date: 2012-03-09
Publication date: 2016-05-04
Anticipated expiration: 2032-03-09
Also published as: EP2685177A1; CN103415747B; WO2012121382A1; JPWO2012121382A1; EP2685177A4; CN103415747A

Description

Technical Field

The present invention relates to a heat pump water heater having a hot water storage tank and a heat pump unit.

Background Art

There have been known heat pump water heaters which employ a circulation heating system to heat water and heat pump water heaters which employ a non-circulation heating system to heat water. In the circulation heating system, heated hot water is supplied to the hot water storage tank at a relatively higher flow rate, whereas in the non-circulation heating system, heated hot water is supplied to a top portion of the hot water storage tank at a relatively lower flow rate. As a refrigerant used in the heat pump unit, there is a refrigerant which condenses in a condenser, such as R410A, R134a, and R407C (hereinafter, referred to as a "refrigerant such as R410A"), or a refrigerant which does not condense in the condenser, such as CO₂ (hereinafter, referred to as a "refrigerant such as CO₂").

Citation List

Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication No. 228258/2002 (Tokukai 2002-228258)
DE 102 57 431 A1 and EP 1 162 419 A1 relate to a hot water supply systems with a heat pump. As for EP 1 162 419 A1 , a control unit controls the operation of an expansion valve based on a temperature difference between a refrigerant temperature at an outlet side of the refrigerant passage in a water heat exchanger and a water temperature at an inlet side in the water heat exchanger. When the expansion valve is controlled in a direction increasing a valve opening degree, the control unit sets an upper limit opening degree of the expansion valve for obtaining a refrigerant pressure corresponding to a target hot water temperature and controls the expansion valve in an opening degree range smaller than the upper limit opening degree. DE 10 2009 017 423 A1 relates to a method for controlling a hot water reservoir. It is based on a CO₂ heat pump for heating process water. It comprises a primary and a secondary water circuit with respective circulation pumps. The parameter for controlling the second circulation pump depends on the working point of the CO₂ heat pump. WO 2009/069734 A1 is directed to a hot water supply apparatus which can avoid hot water shortage but does not perform any useless boiling run with the help of a calorie data storage means. With the help of a switching means hot water can be either supplied to the bottom or the top of a tank.

Summary of Invention

Technical Problem

In a heat pump water heater, hot water heated by a heat pump unit is supplied to a hot water storage tank. When the flow rate of the supplied hot water is high, the heat exchange efficiency is higher than that at a lower flow rate, and therefore the COP (coefficient of performance) of a heat pump is improved. Meanwhile, when the flow rate of hot water is high, hot water in the hot water storage tank is stirred, so that an entering water temperature is increased, which possibly causes a decrease in the COP of the heat pump. Between the circulation heating system and the non-circulation heating system, there is a difference in the flow rate of hot water supplied to the hot water storage tank, and this causes a difference in the COP of the heat pump.
Further, the COP decreases both in the circulation heating system and the non-circulation heating system, for example, with an increase in temperature of hot water supplied from the storage tank to a condenser (entering water temperature). When a refrigerant such as CO₂ is used, the COP is higher in the non-circulation heating system than in the circulation heating system irrespective of the entering water temperature. Therefore, the non-circulation heating system is used in the case of the refrigerant such as CO₂, thereby a higher COP is provided compared with the circulation heating system.
On the other hand, when a refrigerant such as R410A is used, the magnitude relationship between the COP in the circulation heating system and the COP in the non-circulation heating system changes depending on the entering water temperature. Therefore, if the non-circulation heating system is used for the refrigerant such as R410A, the COP is lower than that in the circulation heating system for some entering water temperatures. Accordingly, if the non-circulation heating is solely used for the refrigerant such as R410A, there is a problem that the COP of the heat pump is considerably low for some entering water temperatures.
Therefore, an object of the present invention is to provide a heat pump water heater which is capable of improving the COP (energy efficiency) of a heat pump when a refrigerant which condenses in a condenser is used.

Solution to Problem

According to a first aspect of the present invention, a heat pump water heater includes the features of claim 1.
In this heat pump water heater, the flow rate of hot water supplied to the hot water storage tank is changed with the change in temperature of the refrigerant in the refrigerant circuit or the change in temperature of hot water in the hot water circuit. Therefore, the energy efficiency of the heat pump water heater is improved by changing the flow rate in view of an increase in efficiency and a decrease in efficiency due to an increase in flow rate. Further, the flow rate of hot water supplied to the hot water storage tank is changed by switching the operation mode either to the circulation heating mode or to the non-circulation heating mode. Therefore, even if the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed due to an increase/decrease in the temperature of the refrigerant or in the temperature of hot water, the operation is performed in the mode which provides a higher COP, and thereby the energy efficiency of the heat pump water heater is improved.
According to a second aspect of the present invention, in the heat pump water heater of the first aspect, the flow rate of hot water supplied to the hot water storage tank is changed based on the temperature of the refrigerant in the refrigerant circuit or the temperature of hot water in the hot water circuit which connects the condenser with the hot water storage tank.
In this heat pump water heater, the energy efficiency of the heat pump water heater is improved by changing the flow rate of hot water supplied to the storage tank based on the temperature of the refrigerant in the refrigerant circuit or the temperature of hot water in the hot water circuit.
According to a third aspect of the present invention, in the heat pump water heater of the first or second aspect, the flow rate of hot water supplied to the hot water storage tank is changed based on a period of time during which a heating operation is performed.
In this heat pump water heater, the energy efficiency of the heat pump water heater is improved by changing the flow rate of hot water supplied to the hot water storage tank based on the period of time during which the heating operation is performed.
According to a fourth aspect of the present invention, the heat pump water heater of the first aspect further includes a valve mechanism which changes a state between (i) a first state in which heated hot water is supplied to the hot water storage tank via a first return port provided to a bottom portion of the hot water storage tank and (ii) a second state in which heated hot water is supplied to the hot water storage tank via a second return port provided to the top portion of the hot water storage tank; and the valve, mechanism is switched to provide the first state in the circulation heating mode, and the valve mechanism is switched to provide the second state in the non-circulation heating mode.
In this heat pump water heater, heated hot water is supplied to the bottom portion of the hot water storage tank at a relatively higher flow rate in the circulation heating mode, and therefore the hot water in the hot water storage tank is stirred due to the natural convection (convection due to the difference in temperature), whereas, in the non-circulation heating mode, heated hot water is supplied to the top portion of the hot water storage tank at a relatively lower flow rate, and therefore the hot water in the hot water storage tank is not stirred.
According to a fifth aspect of the present invention, in the heat pump water heater of the first or the fourth aspect, the operation mode is switched to the circulation heating mode when a temperature of hot water supplied from the hot water storage tank to the condenser is lower than a threshold value, and the operation mode is switched to the non-circulation heating mode when the temperature of hot water supplied from the hot water storage tank to the condenser is equal to or higher than the threshold value.
In this heat pump water heater, when the temperature of hot water supplied to the condenser is low, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the temperature of hot water supplied to the condenser is high, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
According to a sixth aspect, in the heat pump water heater of the first or fourth aspect, the operation mode is switched to the circulation heating mode when a temperature of the refrigerant at a middle portion of a refrigerant pipe of the condenser is lower than a threshold value, and the operation mode is switched to the non-circulation heating mode when the temperature of the refrigerant at the middle portion of the refrigerant pipe of the condenser is equal to or higher than the threshold value.
In this heat pump water heater, when the temperature of the refrigerant at the middle portion of the condenser is low, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the temperature of the refrigerant at the middle portion of the condenser is high, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
According to an seventh aspect of the present invention, in the heat pump water heater of the first or fourth aspect, the operation mode is switched to the non-circulation heating mode when a temperature of the refrigerant in the vicinity of an outlet of a refrigerant pipe of the condenser is lower than a threshold value, and the operation mode is switched to the circulation heating mode when the temperature of the refrigerant in the vicinity of the outlet of the refrigerant pipe of the condenser is equal to or higher than the threshold value.
In this heat pump water heater, when the temperature of the refrigerant in the vicinity of the outlet of the condenser is low, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the temperature of the refrigerant in the vicinity of the outlet of the condenser is high, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
According to a eighth aspect of the present invention, in the heat pump water heater of the first or fourth aspect, the operation mode is switched to the non-circulation heating mode when a difference between a temperature of the refrigerant at a middle portion of a refrigerant pipe of the condenser and a temperature of the refrigerant in the vicinity of an outlet of the refrigerant pipe of the condenser is smaller than a threshold value, and the operation mode is switched to the circulation heating mode when the difference is equal to or larger than threshold value.
In this heat pump water heater, when the difference between the temperature of the refrigerant at the middle portion of the condenser and that in the vicinity of the outlet of the condenser is small, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the above difference in temperature is large, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
According to a ninth aspect of the present invention, in the heat pump water heater of any one of the fifth to eighth aspects, the threshold value is changed based on a temperature of air sucked into an evaporator of the refrigerant circuit.
In this heat pump water heater, since the energy efficiency changes depending on the temperature of air sucked into the evaporator both in the circulation heating mode and in the non-circulation heating mode, the threshold value based on which the mode is switched is changed depending on that temperature, and this further improves the energy efficiency.
According to an tenth aspect of the present invention, in the heat pump water heater of any one of the first and fourth to ninth aspects, the second flow rate in the non-circulation heating mode is changed so that the larger a difference between a target temperature to which hot water stored in the hot water storage tank is desired to be heated and the temperature of hot water supplied to the condenser is, the lower the second flow rate
In this heat pump water heater, the flow rate of hot water supplied to the hot water storage tank is decreased, in the non-circulation heating mode, with an increase in the difference between the target temperature and the temperature of hot water supplied to the condenser, and thereby the temperature of hot water supplied to the hot water storage tank approaches the target temperature.
As described above, the present invention provides the following advantageous effects.

Advantageous Effects of Invention

In the first aspect, the flow rate of hot water supplied to the hot water storage tank is changed with the change in temperature of the refrigerant in the refrigerant circuit or the change in temperature of hot water in the hot water circuit. Therefore, the energy efficiency of the heat pump water heater is improved by changing the flow rate in view of an increase in efficiency and a decrease in efficiency due to the increase in flow rate. Further, the flow rate of hot water supplied to the hot water storage tank is changed by switching the operation mode either to the circulation heating mode or to the non-circulation heating mode. Therefore, even if the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed due to an increase/decrease in the temperature of the refrigerant or in the temperature of hot water, the operation is performed in the mode which provides a higher COP, and thereby the energy efficiency of the heat pump water heater is improved.
In the second aspect, the energy efficiency of the heat pump water heater is improved by changing the flow rate of hot water supplied to the storage tank based on the temperature of the refrigerant in the refrigerant circuit or the temperature of hot water in the hot water circuit.
In the third aspect, the energy efficiency of the heat pump water heater is improved by changing the flow rate of hot water supplied to the hot water storage tank based on the period of time during which the heating operation is performed.
In the fourth aspect, heated hot water is supplied to the bottom portion of the hot water storage tank at a relatively higher flow rate in the circulation heating mode, and therefore the hot water in the hot water storage tank is stirred due to the natural convection (convection due to the difference in temperature), whereas, in the non-circulation heating mode, heated hot water is supplied to the top portion of the hot water storage tank at a relatively lower flow rate, and therefore the hot water in the hot water storage tank is not stirred.
In the fifth aspect, when the temperature of hot water supplied to the condenser is low, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the temperature of hot water supplied to the condenser is high, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
In the sixth aspect, when the temperature of the refrigerant at the middle portion of the condenser is low, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the temperature of the refrigerant at the middle portion of the condenser is high, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
In the seventh aspect, when the temperature of the refrigerant in the vicinity of the outlet of the condenser is low, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the circulation heating mode, whereas when the temperature of the refrigerant in the vicinity of the outlet of the condenser is high, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
In the eighth aspect, when the difference between the temperature of the refrigerant at the middle portion of the condenser and that in the vicinity of the outlet of the condenser is large, the circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, whereas when the above difference in temperature is small, the non-circulation heating mode which provides higher energy efficiency is selected between the circulation heating mode and the non-circulation heating mode, and therefore, the efficiency is improved.
In the ninth aspect, since the energy efficiency changes depending on the temperature of air sucked into the evaporator both in the circulation heating mode and the non-circulation heating mode, the threshold value based on which the mode is switched is changed depending on that temperature, and this further improves the energy efficiency.
In the tenth aspect, the flow rate of hot water supplied to the hot water storage tank is decreased, in the non-circulation heating mode, with an increase in the difference between the target temperature and the temperature of hot water supplied to the condenser, and thereby the temperature of hot water supplied to the hot water storage tank approaches the target temperature.

Brief Description of Drawings

[FIG. 1] FIG. 1 is a diagram showing piping of a heat pump water heater of First Embodiment of the present invention.
[FIG. 2] FIG. 2 is a control block diagram of the heat pump water heater of FIG. 1.
[FIG. 3] FIG. 3 is a graph showing a relation between an entering water temperature and a COP.
[FIG. 4] FIG. 4 is a graph showing the entering water temperature and the COP, for comparing cases in which the outside air temperature is different.
[FIG. 5] FIG. 5 is a control block diagram of a heat pump water heater of Second Embodiment of the present invention.
[FIG. 6] FIG. 6 is a graph showing a relation between a middle temperature of a condenser and the COP.
[FIG. 7] FIG. 7 is a control block diagram of a heat pump water heater of Third Embodiment of the present invention.
[FIG. 8] FIG. 8 is a graph showing a relation between an outlet temperature of the condenser and the COP.
[FIG. 9] FIG. 9 is a control block diagram of a heat pump water heater of Fourth embodiment of the present invention.
[FIG. 10] FIG. 10 is a graph showing a difference between the middle temperature and the outlet temperature of the condenser and the COP.
[FIG. 11] FIG. 11 is a diagram showing piping of a heat pump water heater of a modification of First Embodiment of the present invention.

Description of Embodiments

The following describes a heat pump water heater 1 of First Embodiment of the present invention.
As shown in FIG. 1, the heat pump water heater 1 of this embodiment includes: a heat pump unit 2; a hot water storage tank 5; a circulation pump 6; a three way valve (valve mechanism) 7; and a controller 10 (see FIG. 2). The hot water storage tank 5 is connected to a hot water supply terminal A and a water supply source B. The heat pump water heater 1 is configured to perform: a heating operation in which hot water to be stored in the hot water storage tank 5 is heated up; and a hot water supply operation in which hot water in the hot water storage tank 5 is supplied to the hot water supply terminal A. In the hot water supply operation, hot water is supplied from the hot water storage tank 5 to the hot water supply terminal A, and water is supplied from the water supply source B to the hot water storage tank 5. In the heating operation, hot water is taken from the hot water storage tank 5, and the thus taken hot water is heated in a condenser 22 of the heat pump unit 2, and then returned to the hot water storage tank 5. A target temperature to which the hot water stored in the hot water storage tank 5 is desired to be heated ("heating target temperature", for example, 65 degrees Celsius) is set using a not-shown remote control.
The heat pump unit 2 has a refrigerant circuit 3 in which a refrigerant circulates. In the refrigerant circuit 3, a compressor 21, the condenser 22, an expansion valve 23, and an evaporator 24 are provided in this order. Further, a fan 25 is disposed in the vicinity of the evaporator 24. As the refrigerant which circulates in the refrigerant circuit 3, a refrigerant which condenses (changes into liquid) in the condenser 22 is used, such as R410A, R143a, R32, or the like.
In the refrigerant circuit 3, when the compressor 21 is started, a low-pressure gas refrigerant is sucked into the compressor 21, and the refrigerant is compressed in the compressor 21, to be a high-temperature and high-pressure gas refrigerant. Then, in the condenser 22, the high-temperature and high-pressure gas refrigerant exchanges heat with hot water sent from the hot water storage tank 5, and thereby the refrigerant is cooled and condensed. The condensed refrigerant is depressurized in the expansion valve 23, and then heated in the evaporator 24 by heat exchange with air, thereby to evaporate into a low-pressure gas refrigerant again, which returns back to the compressor 21.
A middle temperature sensor 31 for detecting the temperature of the refrigerant is attached to a middle portion of a refrigerant pipe of the condenser 22. An outlet temperature sensor 32 for detecting the temperature of the refrigerant having passed through the condenser 22 is attached in the vicinity of an outlet of the condenser 22 in the refrigerant circuit 3. Further, the heat pump unit 2 includes an outside air temperature sensor 33. The outside air temperature sensor 33 detects the temperature of air sucked into the evaporator 24.
The hot water storage tank 5 is connected to the condenser 22 of the heat pump unit 2 via a hot water circuit 4. A first return port 5a, a discharge port 5b, and a water supply port 5c are provided at the bottom of the hot water storage tank 5, and a second return port 5d and a hot water outlet port 5e are provided at the top of the hot water storage tank 5.
The first return port 5a is connected with a return branch pipe 4c, and the second return port 5d is connected with a return branch pipe 4d. The return branch pipes 4c and 4d constitute a part of the hot water circuit 4. The return branch pipes 4c and 4d are connected to a return pipe 4b via a three way valve 7. The return pipe 4b is connected to the condenser 22.
The three way valve 7 changes a state between: a first state in which the return pipe 4b communicates with the return branch pipe 4c; and a second state in which the return pipe 4b communicates with the return branch pipe 4d. When the three way valve 7 is in the first state, hot water heated in the condenser 22 is supplied to the hot water storage tank 5 via the first return port 5a, whereas, when the three way valve 7 is in the second state, hot water heated in the condenser 22 is supplied to the hot water storage tank 5 via the second return port 5d. An exiting hot water temperature sensor 42 for detecting the temperature of hot water having passed through the condenser 22 is attached to the return pipe 4b.
The discharge port 5b is connected with a forward pipe 4a extending to the condenser 22. The forward pipe 4a constitutes a part of the hot water circuit 4. The forward pipe 4a is provided with: a circulation pump 6 inserted therein; and an entering water temperature sensor 41 for detecting the temperature of hot water entering into the condenser 22 (hereinafter, referred to as an "entering water temperature").
The circulation pump 6 is provided for circulating hot water in the hot water circuit 4 during the heating operation, and to adjust the flow rate of hot water supplied to the hot water storage tank 5. When the circulation pump 6 is driven, hot water (or cold water) in the hot water storage tank 5 is drawn into the forward pipe 4a through the discharge port 5b, and after heated in the condenser 22, the water is returned back to the hot water storage tank 5 via the first return port 5a or the second return port 5d. Further, the flow rate of hot water supplied to the hot water storage tank 5 is increased by increasing the number of revolutions of the circulation pump 6, whereas the flow rate is decreased by decreasing the number of revolutions of the circulation pump 6.
The water supply port 5c is connected with the water supply source B, and the hot water outlet port 5e is connected with the hot water supply terminal A. In the hot water supply operation, high-temperature hot water exits through the hot water outlet port 5e of the hot water storage tank 5, and low-temperature water enters through the water supply port 5c of the hot water storage tank 5.
A tank temperature sensor 43 is attached on an outer surface of the hot water storage tank 5. The tank temperature sensor 43 detects the temperature of hot water in the hot water storage tank 5. The tank temperature sensor 43 is provided at a substantially middle portion of the hot water storage tank 5 with respect to a vertical direction. Since high-temperature hot water exits through the hot water outlet port 5e of the hot water storage tank 5 and low-temperature water enters through the water supply port 5c of the hot water storage tank 5 in the hot water supply operation, hot water closer to the top in the hot water storage tank 5 has a higher temperature, while hot water closer to the bottom has a lower temperature. The more hot water supply operation is performed, the more the space occupied by the low-temperature hot water expands from the bottom toward the top. Therefore, the temperature detected by the tank temperature sensor 43 indicates how much high-temperature hot water remains in the hot water storage tank 5 (that is, indicates the amount of remaining hot water).
The heat pump water heater 1 of this embodiment has two operation modes for the heating operation, which are a circulation heating mode and a non-circulation heating mode. In the heat pump water heater 1 of this embodiment, the operation mode is switched between the circulation heating mode and the non-circulation heating mode, based on the temperature of hot water supplied from the hot water storage tank 5 to the condenser 22 (the entering water temperature).
The circulation heating mode is a mode in which the three way valve 7 is switched to provide the first state, and thereby hot water heated in the condenser 22 is supplied to a bottom portion of the hot water storage tank 5 via the first return port 5a. In this embodiment, the flow rate of hot water supplied to the hot water storage tank 5 in the circulation heating mode is a first flow rate which is relatively higher and predetermined flow rate. Further, the supply of high-temperature hot water to the bottom portion of the hot water storage tank 5 causes convection in the hot water storage tank 5 due to a difference in temperature, and therefore hot water in the hot water storage tank 5 is stirred, with the result that the temperature of hot water in the hot water storage tank 5 is substantially homogenized. Therefore, with continuous operation in the circulation heating mode, the temperature of hot water in the entire hot water storage tank 5 gradually increases.
The non-circulation heating mode is a mode in which the three way valve 7 is switched to provide the second state, and thereby hot water is supplied to a top portion of the hot water storage tank 5 at a flow rate lower than the first flow rate. In the non-circulation heating mode, high-temperature hot water is supplied at the lower flow rate to the top portion of the hot water storage tank 5, and therefore convection due to the difference in temperature is not caused in the hot water storage tank 5. In this embodiment, the flow rate of hot water supplied to the hot water storage tank 5 in the non-circulation heating mode is a second flow rate which is lower than the first flow rate, and the flow rate is changed based on the heating target temperature. At the second flow rate, hot water in the hot water storage tank 5 is hardly stirred. Thus, in the non-circulation heating mode, high-temperature hot water is supplied to the top portion of the hot water storage tank 5 so that the hot water therein is not stirred, and therefore, with continuous operation in the non-circulation heating mode, the space occupied by the high-temperature hot water expands from the top toward the bottom of the hot water storage tank 5.
Next, description will be given for a threshold value of the entering water temperature at which the operation mode is switched between the circulation heating mode and the non-circulation heating mode. FIG. 3 is an example of a graph showing the relation between the entering water temperature and the COP. The COP (coefficient of performance) is a value representing heating capacity for 1 kW of power consumption, and it provides a measure of energy efficiency. The higher the COP is, the higher the energy efficiency is.
In the example of FIG. 3, R410A is used as a refrigerant, and the temperature of outside air ("outside air temperature") is Ta. As shown in FIG. 3, both in the circulation heating mode and the non-circulation heating mode, the COP decreases with an increase in the entering water temperature; however, the decrease is gentler in the non-circulation heating mode than in the circulation heating mode. Further, the curve for the circulation heating mode crosses the curve for the non-circulation heating mode. When the entering water temperature is lower than a threshold value T_A, the COP in the circulation heating mode is higher than that in the non-circulation heating mode; whereas when the entering water temperature is higher than the threshold value T_A, the COP in the non-circulation heating mode is higher than that in the circulation heating mode.
The reason why the COP in the circulation heating mode is higher than that in the non-circulation heating mode when the entering water temperature is lower than the threshold value T_A is as follows: since the entering water temperature is low, the degree of increase in efficiency caused by high heat exchange efficiency resulting from the high flow rate (increase in efficiency due to the increase in flow rate) exceeds the degree of decrease in efficiency caused by an increase in the entering water temperature resulting from the high flow rate (decrease in efficiency due to the increase in flow rate).
On the other hand, the reason why the COP in the non-circulation heating mode is higher than that in the circulation heating mode when the entering water temperature is higher than threshold value T_A is as follows: since the entering water temperature is high, the degree of decrease in efficiency caused by an increase in the entering water temperature resulting from the high flow rate (decrease in efficiency due to the increase in flow rate) exceeds the degree of increase in efficiency caused by high heat exchange efficiency resulting from the high flow rate (increase in efficiency due to the increase in flow rate).
When the heating operation is started, the operation is performed in the circulation heating mode since the entering water temperature is lower than the threshold value T_A. Thereafter, when the entering water temperature exceeds the threshold value T_A as a result of the heating operation, the operation mode is switched to the non-circulation heating mode. With this, the operation is performed in the mode which provides a higher COP.
FIG. 4 shows how the relation between the entering water temperature and the COP changes when the outside air temperature falls from Ta to Ta' (Ta' < Ta) under the conditions of FIG. 3. As shown in FIG. 4, the lower the outside air temperature is, the lower the COP is, both in the circulation heating mode and the non-circulation heating mode. Therefore, the threshold value T_A of the entering water temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed is decreased to T_A', along with the decrease in the outside air temperature.
In the heat pump water heater 1 of this embodiment, when the outside air temperature is Ta, the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the threshold value T_A of the entering water temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed. When the entering water temperature detected by the entering water temperature sensor 41 is lower than the threshold value T_A, the heating operation is performed in the circulation heating mode, and when the entering water temperature is equal to or higher than the threshold value T_A, the heating operation is performed in the non-circulation heating mode. Thus, by switching the operation mode between the circulation heating mode and the non-circulation heating mode based on the entering water temperature, the COP is improved compared with the case where the heating operation is performed only in the circulation heating mode or the case where the heating operation is performed only in the non-circulation heating mode.
Further, as shown in FIG. 4, the threshold value T_A of the entering water temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed changes depending on the outside air temperature, and therefore, in the heat pump water heater 1 of this embodiment, the threshold value T_A of the entering water temperature at which the operation mode is switched is changed depending on the outside air temperature. To be more specific, the threshold value T_A is lowered as the outside air temperature decreases.
Next, the controller 10 will be described. As shown in FIG. 2, the controller 10 includes: a threshold value determiner 11; a mode switcher 12; a three way valve controller 13; and a circulation pump controller 14.
The threshold value determiner 11 determines the threshold value T_A of the entering water temperature based on the outside air temperature detected by the outside air temperature sensor 33. The threshold value determiner 11 stores a plurality of threshold values T_A of the entering water temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed so that the threshold values T_A respectively correspond to various outside air temperatures. Therefore, the threshold value determiner 11 selects one threshold value T_A corresponding to the detected outside air temperature among the threshold values T_A stored therein, and thereby determines the threshold value T_A.
The mode switcher 12 switches the operation mode between the circulation heating mode and the non-circulation heating mode in the heating operation, based on the entering water temperature detected by the entering water temperature sensor 41 and the threshold value T_A determined by the threshold value determiner 11. When the entering water temperature is lower than the threshold value T_A, the operation mode is switched to the circulation heating mode, whereas when the entering water temperature is equal to or higher than the threshold value T_A, the operation mode is switched to the non-circulation heating mode.
The three way valve controller 13 switches the three way valve 7 in the heating operation, in accordance with the mode determined by the mode switcher 12. In the case of the circulation heating mode, the three way valve 7 is switched to provide the first state, whereas in the case of the non-circulation heating mode, the three way valve 7 is switched to provide the second state.
The circulation pump controller 14 controls the number of revolutions of the circulation pump 6 in the heating operation, in accordance with the mode determined by the mode switcher 12. In the case of the circulation heating mode, the number of revolutions of the circulation pump 6 is controlled so that the flow rate of hot water supplied to the hot water storage tank 5 becomes equal to the predetermined first flow rate. Meanwhile, in the case of the non-circulation heating mode, the number of revolutions of the circulation pump 6 is controlled based on the difference between the entering water temperature and the heating target temperature. The larger the difference between the entering water temperature and the heating target temperature is, the smaller the number of revolutions of the circulation pump 6 is. In the non-circulation heating mode, the circulation pump controller 14 controls the number of revolutions of the circulation pump 6 within a range in which the flow rate of hot water supplied to the hot water storage tank 5 does not exceed the second flow rate, which is lower than the first flow rate, and at which flow rate hot water in the hot water storage tank 5 is hardly stirred, as described above.
In the heat pump water heater 1 of this embodiment, the operation mode is switched to either the circulation heating mode or the non-circulation heating mode, based on the entering water temperature, which is the temperature of water supplied to the condenser 22. Therefore, even if the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed due to an increase/decrease in the entering water temperature, the operation is performed in the mode which provides a higher COP, and thereby the energy efficiency of the heat pump water heater is improved.
In this embodiment, when the entering water temperature of the water supplied to the condenser 22 is lower than the threshold value T_A, the operation is performed in the circulation heating mode which provides a higher COP between the circulation heating mode and the non-circulation heating mode, whereas when the entering water temperature of the water supplied to the condenser 22 is equal to or higher than the threshold value T_A, the operation is performed in the non-circulation heating mode which provided a higher COP between the circulation heating mode and the non-circulation heating mode. Therefore, the efficiency is improved.
In this embodiment, since the COP changes depending on the outside air temperature (the temperature of air sucked into the evaporator 24) in both of the circulation heating mode and the non-circulation heating mode, the threshold value T_A based on which the operation mode is switched is changed depending on the detected outside air temperature, and this further improves the COP.
In this embodiment, heated hot water is supplied to the bottom portion of the hot water storage tank 5 at a relatively higher flow rate in the circulation heating mode, and therefore the hot water in the hot water storage tank 5 is stirred due to the natural convection (convection due to the difference in temperature), whereas, in the non-circulation heating mode, heated hot water is supplied to the top portion of the hot water storage tank 5 at a relatively lower flow rate, and therefore the hot water in the hot water storage tank 5 is not stirred.
In this embodiment, the flow rate of hot water supplied to the hot water storage tank 5 is decreased with an increase in the difference between the entering water temperature and the heating target temperature in the non-circulation heating mode, and thereby the temperature of hot water supplied to the hot water storage tank 5 approaches the heating target temperature.

The following describes Second Embodiment of the present invention.
A heat pump water heater 101 of this embodiment is different from that of First Embodiment in that the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the temperature of the refrigerant at the middle portion of the refrigerant pipe of the condenser 22 (hereinafter, referred to as a "middle temperature"). Except for that mentioned above, the heat pump water heater 101 has the same structure as that of First Embodiment. Components having the same structure as those in First Embodiment will be given the same reference numerals, and the description thereof will be omitted, if appropriate.
As shown in FIG. 5, a controller 110 of the heat pump water heater 101 of this embodiment includes: a threshold value determiner 111; a mode switcher 112; the three way valve controller 13; and the circulation pump controller 14.
FIG. 6 is an example of a graph showing the relation between the middle temperature and the COP. As shown in FIG. 6, the curve for the circulation heating mode crosses the curve for the non-circulation heating mode. When the middle temperature is lower than a threshold value T_B, the COP is higher in the circulation heating mode than in the non-circulation heating mode, and when the middle temperature is equal to or higher than the threshold value T_B, the COP is higher in the non-circulation heating mode than in the circulation heating mode.
In the heat pump water heater 101 of this embodiment, the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the threshold value T_B of the middle temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed. When the temperature detected by the middle temperature sensor 31 is lower than the threshold value T_B, the heating operation is performed in the circulation heating mode, whereas when the middle temperature is equal to or higher than the threshold value T_B, the heating operation is performed in the non-circulation heating mode. Thus, by switching the operation mode between the circulation heating mode and the non-circulation heating mode based on the middle temperature, the COP is improved compared with a case where the heating operation is performed only in the circulation heating mode or a case where the heating operation is performed only in the non-circulation heating mode.
The threshold value determiner 111 determines the threshold value T_B of the middle temperature based on the outside air temperature detected by the outside air temperature sensor 33. As is in First Embodiment, the threshold value determiner 111 stores a plurality of threshold values T_B of the middle temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed so that the threshold values T_B respectively correspond to various outside air temperatures. Therefore, the threshold value determiner 111 selects one threshold value T_B corresponding the detected outside air temperature among the threshold values T_B stored therein, and thereby determines the threshold value T_B.
The mode switcher 112 switches the operation mode between the circulation heating mode and the non-circulation heating mode in the heating operation, based on the middle temperature detected by the middle temperature sensor 31 and the threshold value T_B determined by the threshold value determiner 111. When the middle temperature is lower than the threshold value T_B, the mode switcher 112 switches the operation mode to the circulation heating mode, whereas when the middle temperature is equal to or higher than the threshold value T_B, the mode switcher 112 switches the operation mode to the non-circulation heating mode.
In the heat pump water heater 101 of this embodiment, the operation mode is switched to either the circulation heating mode or the non-circulation heating mode based on the middle temperature in the condenser 22. Therefore, even if the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed due to an increase/decrease in the middle temperature, the operation is performed in the mode which provides a higher COP, and thereby the energy efficiency of the heat pump water heater is improved.

The following describes Third Embodiment of the present invention.
A heat pump water heater 201 of this embodiment is different from that of First Embodiment in that the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the temperature of the refrigerant in the vicinity of the outlet of the refrigerant pipe of the condenser 22 (hereinafter, referred to as an "outlet temperature"). Except for that mentioned above, the heat pump water heater 201 has the same structure as that of First Embodiment. Components having the same structure as those in First Embodiment will be given the same reference numerals, and the description thereof will be omitted, if appropriate.
As shown in FIG. 7, a controller 210 of the heat pump water heater 201 of this embodiment includes: a threshold value determiner 211; a mode switcher 212; the three way valve controller 13; and the circulation pump controller 14.
FIG. 8 is an example of a graph showing the relation between the outlet temperature and the COP. As shown in FIG. 8, the curve for the circulation heating mode crosses the curve for the non-circulation heating mode. When the outlet temperature is lower than a threshold value T_C, the COP is higher in the non-circulation heating mode than in the circulation heating mode, and when the outlet temperature is equal to or higher than the threshold value T_C, the COP is higher in the circulation heating mode than in the non-circulation heating mode.
In the heat pump water heater 201 of this embodiment, the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the threshold value T_C of the outlet temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed. When the temperature detected by the outlet temperature sensor 32 is lower than the threshold value T_C, the heating operation is performed in the non-circulation heating mode, whereas when the outlet temperature is equal to or higher than the threshold value T_C, the heating operation is performed in the circulation heating mode. Thus, by switching the operation mode between the circulation heating mode and the non-circulation heating mode based on the outlet temperature, the COP is improved compared with a case where the heating operation is performed only in the circulation heating mode or a case where the heating operation is performed only in the non-circulation heating mode.
The threshold value determiner 211 determines the threshold value T_C of the outlet temperature based on the outside air temperature detected by the outside air temperature sensor 33. As is in First Embodiment, the threshold value determiner 211 stores a plurality of threshold values T_c of the outlet temperature at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed so that the threshold values T_c respectively correspond to various outside air temperatures. Therefore, the threshold value determiner 211 selects one threshold value T_c corresponding to the detected outside air temperature among the threshold values T_c stored therein, and thereby determines the threshold value T_c.
The mode switcher 212 switches the operation mode between the circulation heating mode and the non-circulation heating mode in the heating operation, based on the outlet temperature detected by the outlet temperature sensor 32, and the threshold value T_c determined by the threshold value determiner 211. When the outlet temperature is lower than the threshold value T_c, the mode switcher 212 switches the operation mode to the non-circulation heating mode, whereas when the outlet temperature is equal to or higher than the threshold value T_c, the mode switcher 212 switches the operation mode to the circulation heating mode.
In the heat pump water heater 201 of this embodiment, the operation mode is switched to either the circulation heating mode or the non-circulation heating mode based on the outlet temperature of the condenser 22. Therefore, even if the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed due to an increase/decrease in the outlet temperature, the operation is performed in the mode which provides a higher COP, and thereby the energy efficiency of the heat pump water heater is improved.

The following describes Fourth embodiment of the present invention.
A heat pump water heater 301 of this embodiment is different from that of First Embodiment in that the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the difference between the temperature of the refrigerant in the vicinity of the outlet of the condenser 22 and the temperature of the refrigerant at the middle portion of the refrigerant pipe of the condenser 22 (hereinafter, referred to as a "difference between the middle temperature and the outlet temperature"). Except for that mentioned above, the heat pump water heater 301 has the same structure as that of First Embodiment. Components having the same structure as those in First Embodiment will be given the same reference numerals, and the description thereof will be omitted, if appropriate.
As shown in FIG. 9, a controller 310 of the heat pump water heater 301 of this embodiment includes: a threshold value determiner 311; a mode switcher 312; the three way valve controller 13; and the circulation pump controller 14.
FIG. 10 is an example of a graph showing the relation between: the difference between the middle temperature and the outlet temperature; and the COP. As shown in FIG. 10, the curve for the circulation heating mode crosses the curve for the non-circulation heating mode. When the difference between the middle temperature and the outlet temperature is smaller than a threshold value T_D, the COP is higher in the non-circulation heating mode than in the circulation heating mode, and when the difference between the middle temperature and the outlet temperature is equal to or larger than the threshold value T_D, the COP is higher in the circulation heating mode than in the non-circulation heating mode.
In the heat pump water heater 301 of this embodiment, the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the threshold value T_D of the difference between the middle temperature and the outlet temperature at which value the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed. When the above difference, which is the difference between the middle temperature detected by the middle temperature sensor 31 and the outlet temperature detected by the outlet temperature sensor 32, is smaller than the threshold value T_D, the heating operation is performed in the non-circulation heating mode, whereas when the difference between the middle temperature and the outlet temperature is equal to or larger than the threshold value T_D, the heating operation is performed in the circulation heating mode. Thus, by switching the operation mode between the circulation heating mode and the non-circulation heating mode based on the difference between the middle temperature and the outlet temperature, the COP is improved compared with a case where the heating operation is performed only in the circulation heating mode or a case where the heating operation is performed only in the non-circulation heating mode.
The threshold value determiner 311 determines the threshold value T_D of the difference between the middle temperature and the outlet temperature based on the outside air temperature detected by the outside air temperature sensor 33. As is in First Embodiment, the threshold value determiner 311 stores a plurality of threshold values T_D of the difference between the middle temperature and the outlet temperature at which values the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed so that the threshold values T_D respectively correspond to various outside air temperatures. Therefore, the threshold value determiner 311 selects one threshold value T_D corresponding the detected outside air temperature among the threshold values T_D stored therein, and thereby determines the threshold value T_D.
The mode switcher 312 switches the operation mode between the circulation heating mode and the non-circulation heating mode in the heating operation, based on: the difference between the middle temperature and the outlet temperature; and the threshold value T_D determined by the threshold value determiner 311. When the difference between the middle temperature and the outlet temperature is smaller than the threshold value T_D, the mode switcher 312 switches the operation mode to the non-circulation heating mode, whereas when the difference between the middle temperature and the outlet temperature is equal to or larger than the threshold value T_D, the mode switcher 312 switches the operation mode to the circulation heating mode.
In the heat pump water heater 301 of this embodiment, the operation mode is switched to either the circulation heating mode or the non-circulation heating mode based on the difference between the middle temperature and the outlet temperature of the condenser 22. Therefore, even if the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed due to an increase/decrease in the difference between the middle temperature and the outlet temperature, the operation is performed in the mode which provides a higher COP, and thereby the energy efficiency of the heat pump water heater is improved.
While the embodiments of the present invention have been described, it should be noted that the scope of the invention is not limited to the above-described embodiments. The scope of the present invention is defined by the appended claims rather than the foregoing description of the embodiments, and the present invention is intended to embrace all alternatives, modifications and variances which fall within the scope of the appended claims.
In the above-described embodiments, the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the entering water temperature, the middle temperature, the outlet temperature, or the difference between the middle temperature and the outlet temperature; however, the present invention is not limited thereto. The operation mode may be switched between the circulation heating mode and the non-circulation heating mode based on a period of time during which the heating operation is performed. Since the period of time during which the heating operation is performed corresponds to the change in the entering water temperature, in the middle temperature, in the outlet temperature, and in the difference between the middle temperature and the outlet temperature, it is possible to switch the operation mode between the circulation heating mode and the non-circulation heating mode in the same manner as in the above embodiments.
In the above-described embodiments, the flow rate of hot water supplied to the hot water storage tank 5 is controlled by controlling the number of revolutions of the circulation pump 6; however, the flow rate of hot water supplied to the hot water storage tank 5 may be controlled by controlling a flow regulating valve provided to the forward pipe 4a or the return pipe 4b.
Further, in each of the above-described embodiments, the threshold value is changed depending on the detected outside air temperature; however, the operation mode may be switched based on a threshold value which is predetermined irrespective of the outside air temperature . For example, the predetermined threshold value may be a threshold value at which the magnitude relationship between the COP in the circulation heating mode and the COP in the non-circulation heating mode is reversed under the condition of an average outside air temperature.
In the circulation heating mode of each of the above embodiments, hot water is supplied to the bottom portion of the hot water storage tank 5; however, hot water may be supplied to the top portion of the hot water storage tank 5. In this modification which is not part of the claimed invention, as shown in FIG. 11, hot water is supplied to the top portion of the hot water storage tank 5 both in the circulation heating mode and in the non-circulation heating mode, and therefore, it is not necessary to control the three way valve 7. Accordingly, there is no need to provide the three way valve 7 and the return branch pipe 4c. Further, in the hot water storage tank 5 of this modification, a stirring prevention member is provided so as to be opposed to the second return port 5d. This prevents hot water in the hot water storage tank 5 from being stirred by hot water supplied through the second return port 5d.
In the circulation heating mode of each of the above embodiments, the flow rate of hot water supplied to the bottom portion of the hot water storage tank 5 (the first flow rate) is always constant; however, the flow rate may be changed based on the heating target temperature or the entering water temperature.
In the above-described embodiments, R410A is used as a refrigerant; however, another refrigerant which does not condense in the condenser such as R134A or R407C may be used.
In the above-described embodiments, the operation mode is switched between the circulation heating mode and the non-circulation heating mode based on the entering water temperature, the middle temperature, the outlet temperature, or the difference between the middle temperature and the outlet temperature; however, the present invention is not limited thereto. The operation mode may be switched between the circulation heating mode and the non-circulation heating mode based on the temperature of the refrigerant at another portion in the refrigerant circuit, or based on the temperature of hot water at another portion in the hot water circuit connecting the condenser with the hot water storage tank.

Industrial Applicability

With the use of the present invention, the energy efficiency is improved.

Reference Signs List

1, 101, 201, 301: Heat pump water heater 2: Heat pump unit
3: Refrigerant circuit
4: Hot water circuit
5: Hot water storage tank
5a: First return port
5d: Second return port
6: Circulation pump
7: Three way valve (valve mechanism)
22: Condenser
24: Evaporator

Claims

A heat pump water heater (1, 101), comprising:
a heat pump unit (2) including a condenser (22), a refrigerant circuit (3) in which a refrigerant condensing in the condenser (22) circulates; and

a hot water storage tank (5) which stores hot water heated in the condenser (22), wherein:
an operation mode is selectable from (i) a circulation heating mode in which heated hot water is supplied to a bottom portion of the hot water storage tank (5) at a first flow rate, and (ii) a non-circulation heating mode in which heated hot water is supplied to a top portion of the hot water storage tank (5) at a second flow rate lower than the first flow rate; and

by switching the operation mode between the circulation heating mode and the non-circulation heating mode the flow rate of hot water supplied to the hot water storage tank (5) is changed with a change in temperature of the refrigerant in the refrigerant circuit (3) or with a change in temperature of hot water in a hot water circuit (4) which connects the condenser (22) with the hot water storage tank (5).
The heat pump water heater (1, 101) according to Claim 1, wherein the flow rate of hot water supplied to the hot water storage tank (5) is changed based on the temperature of the refrigerant in the refrigerant circuit (3) or the temperature of hot water in the hot water circuit (4) which connects the condenser (22) with the hot water storage tank (5).
The heat pump water heater (1, 101) according to Claim 1 or 2, wherein the flow rate of hot water supplied to the hot water storage tank (5) is changed based on a period of time during which a heating operation is performed.
The heat pump water heater (1, 101) according to Claim 1, further comprising a valve mechanism (7) which changes a state between (i) a first state in which heated hot water is supplied to the hot water storage tank (5) via a first return port (5a) provided to a bottom portion of the hot water storage tank (5) and (ii) a second state in which heated hot water is supplied to the hot water storage tank (5) via a second return port (5d) provided to the top portion of the hot water storage tank (5), wherein
the valve mechanism (7) is switched to provide the first state in the circulation heating mode, and the valve mechanism is switched to provide the second state in the non-circulation heating mode.
The heat pump water heater (1, 101) according to Claim 1 or 4, wherein the operation mode is switched to the circulation heating mode when a temperature of hot water supplied from the hot water storage tank (5) to the condenser (22) is lower than a threshold value, and the operation mode is switched to the non-circulation heating mode when the temperature of hot water supplied from the hot water storage tank (5) to the condenser is equal to or higher than the threshold value.
The heat pump water heater (1, 101) according to Claim 1 or 4, wherein the operation mode is switched to the circulation heating mode when a temperature of the refrigerant at a middle portion of a refrigerant pipe of the condenser (22) is lower than a threshold value, and the operation mode is switched to the non-circulation heating mode when the temperature of the refrigerant at the middle portion of the refrigerant pipe of the condenser (22) is equal to or higher than the threshold value.
The heat pump water heater (1, 101) according to Claim 1 or 4, wherein the operation mode is switched to the non-circulation heating mode when a temperature of the refrigerant in the vicinity of an outlet of a refrigerant pipe of the condenser is lower than a threshold value, and the operation mode is switched to the circulation heating mode when the temperature of the refrigerant in the vicinity of the outlet of the refrigerant pipe of the condenser (22) is equal to or higher than the threshold value.
The heat pump water heater (1, 101) according to Claim 1 or 4, wherein the operation mode is switched to the non-circulation heating mode when a difference between a temperature of the refrigerant at a middle portion of a refrigerant pipe of the condenser and a temperature of the refrigerant in the vicinity of an outlet of the refrigerant pipe of the condenser (22) is smaller than a threshold value, and the operation mode is switched to the circulation heating mode when the difference is equal to or larger than threshold value.
The heat pump water heater (1, 101) according to any one of Claims 5 to 8, wherein the threshold value is changed based on a temperature of air sucked into an evaporator of the refrigerant circuit (3).
The heat pump water heater (1, 101) according to any one of Claims 1 and 4 to 9, wherein the second flow rate in the non-circulation heating mode is changed so that the larger a difference between a target temperature to which hot water stored in the hot water storage tank (5) is desired to be heated and the temperature of hot water supplied to the condenser (22) is, the lower the second flow rate is.